Problem: Xanthia can read 100 pages per hour and Molly can read 50 pages per hour. If they each read the same book, and the book has 225 pages, how many more minutes than Xanthia would it take for Molly to finish reading the book?
Reading the book takes Xanthia

$\frac{225}{100}=2.25$ hours.

It takes Molly

$\frac{225}{50}=4.5$ hours.

The difference is $2.25$ hours, or $2.25(60)=\boxed{135}$ minutes.